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Homework Help: Heat flux through wall question

  1. Mar 2, 2015 #1
    1. The problem statement, all variables and given/known data

    I was given this question.

    The wall of a building is constructed of face brick (10cm thick) and concrete block (50 cm thick) with an air filled cavity (5cm thick). The interior and exterior convection coefficients are 8.3 W.m^-2.K^_1 and 35W.m^-2.K^_1 respectively. The brick thermal conductivity is 0.79W.m^-1.K^_1 , the concrete block 1.63W.m^-1.K^_1, and the air cavity 0.26W.m^-1.K^_1. If the ambient air temperature is -10 degrees celsius and the room air temperature is 21 degrees Celsius, what will be the Heat flux through the wall.

    2. Relevant equations
    I know that Rtotal is equal to 1/h-inA + L1/k1A + ... + Ln/knA + 1/h-outA (h= Heat transfer coefficient, k= thermal conductivity, L=length, A=area)

    Heat transfer rate = (t1-t2)/Rtotal

    Heat flux = Heat transfer rate/Area

    3. The attempt at a solution
    The problem is that these are all the values I was given. I wasn't given any dimensions for the wall other than the thickness so I can't Calculate the area. Is this a mistake in the question or am I supposed to do it another way?
    I attempted a solution by filling in all the values given in the relevant places but had to stop when I got to the area.

  2. jcsd
  3. Mar 2, 2015 #2


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    Staff Emeritus
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    Homework Helper

    Why don't you work the problem assuming a unit area for the wall (say 1 m2).
  4. Mar 2, 2015 #3
    The Heat Flux is defined as the heat flow per unit area.

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